WMA files (.WMA) are a common sight for Windows users. This codec was developed by Microsoft as part of its Windows Media suite, and the acronym stands for Windows Media Audio
WMA had a brief heyday as a music download format around 2005-2006, but steadily fell out of fashion as music download stores transitioned to the more universal MP3 format. The continued use of WMA is attributable to the fact that it is still used as the default format for ripping CDs using Windows Media Player.
You'll also see WMA used for subscription music services such as Napster and Rhapsody, since the format lends itself well to copy protection (aka PlaysForSure
WMA files are comparable to AAC files when it comes sound quality and file size, but in general you'll find WMA supported on a wider range of products than AAC. Of course, the one place you won't find WMA supported is the Apple iPod, which is a considerable drawback of the format considering the iPod's popularity.